This invention relates to a telephone set, and more particularly, to a fail-safe digital telephone set.
New digital telephone sets associated with a digital PBX are designed to be compatible with the digital interface requirements of the digital PBX. The digital PBX provides the switching interface and logic between associated digital phones. Since the protocol established by the telephone company for initiating and completing a phone call from one subscriber phone to another via the telephone lines is essentially analog in nature, the digital PBX also provides a digital to analog and analog to digital interface for the associated digital phones when the associated digital phones are to communicate outside the PBX environment utilizing telephone lines provided by the telephone company. Generally power to the associated digital phones are provided by the PBX. When the power from the PBX is lost or PBX has a malfunction, the digital phones become inoperable; hence, communication outside the PBX environment is disabled, thereby losing the capability of calling for the assistance or services of a repairman.
Hence, there is a need to provide at least one digital phone which is capable of communicating outside of the PBX environment in the case of such catastrophic failure. The present invention provides a telephone set which in its normal operating mode is compatible with the digital interface and digital protocol of the PBX system, and which, upon a failure of the PBX, is compatible with the analog interface of the telephone lines thereby permitting communication outside of the PBX environment in the failure mode.